1. Field of the Invention
The present invention relates to a technology for correcting color misalignment of an image in an image forming apparatus.
2. Description of the Related Art
A color image forming apparatus forms a full color image by superimposing images in different colors. If positions of the images in different colors are shifted from the preset position, an obtained image such as a line image or a text image is not in a desired color or experiences a color drifting or a color shading, resulting in degradation of image quality. Therefore, it is necessary to adjust the positions of the images in different colors upon forming an image in the color image forming apparatus. A conventional technology for correcting a position shift between images in different colors caused by change in an ambient temperature, an device-internal temperature, or the like in the image forming apparatus having a plurality of photosensitive elements is disclosed in, for example, Japanese Patent Application Laid-Open No. 2004-295083.
In the above conventional technology, a position-shift correction pattern for each color is formed on a transfer belt, and a plurality of sensors are configured to detect the patterns. Then, the amount of shift, such as a magnification error in the main-scanning direction and a registration error in the main-scanning direction and in the sub-scanning direction, is detected based on a signal from the sensor to correct the shift. With the detection of the above parameters, not only a position shift caused by the environmental change but also a position shift caused by temporal change can be corrected. As a result, an image in desired quality can be formed without color shift.
In the conventional technology, the position shift caused by the environmental change and the position shift caused by temporal change are corrected by detecting amounts of position shift of images in other colors with respect to a position of an image in a reference color. However, there is a possibility that the position of the image in the reference color is also shifted due to the environmental change and the temporal change, shifting the reference position for the images in other colors. For example, even if the reference position is adjusted at the time of shipping from a factory, the reference position may change after delivery, or the reference position may be shifted from a position for the first image forming after forming a plurality of images. The same goes even for an image forming apparatus that forms a monochrome (black-and-white) image.
When forming the position-shift correction patterns for respective colors to correct the position shift between images during a continuous printing, it is necessary to form the patterns within a predetermined time (distance). In some cases, the patterns for all colors may not be formed because the predetermined time is limited. However, it is not preferable to lengthen the time (distance) considering an overall printing speed.